The invention relates to an apparatus for prewarming heating oil ahead of the nozzle of a burner having a PTC resistor with electric current flowing through it which is in heat-conductive contact with a line carrying the heating oil to the nozzle.
Oil burners of low and minimum output have substantial advantages in many applications. With burners of this kind, it is possible to adapt the heat output to relatively low requirements as well, such as are found in heating systems for single floors of a building or for single rooms. The low burner output makes it possible to use a smaller container, which thus is less expensive and saves space. The heat insulation of the container is more favorable, and temperature regulation of the container can be attained with fewer startups of the burner, with the result that there is less soiling of the burner and less impact on the environment.
The essential problem with oil burners of minimum output resides in the small cross sections of the nozzle ducts. The narrowness of the nozzle ducts produces poor consistency in the oil throughput and frequently causes the ducts to become plugged.
It is known to repond to these disadvantages by prewarming the heating oil ahead of the nozzle. This prewarming reduces the viscosity of the oil, and satisfactory atomization can be attained at a lower atomization pressure. The lower pressure causes a reduced oil throughput and a lower burner output. In addition, the reduced viscosity lessens the danger of plugging. On the other hand, if it is not desired to reduce the oil throughput and accordingly the burner output, then the cross section of the nozzle ducts can be enlarged because of the reduced atomization pressure. In this case, a substantial lessening of the danger of plugging, and thus an increase in the reliability of the burner, are attained.
To prewarm the heating oil, it is known to use an electric resistance heating means. Electric resistance heating has the disadvantage of requiring a large amount of space. A still more serious disadvantage is that electric resistance heating can cause overheating of the oil beyond the optimum temperature, which may be 70.degree.-80.degree. C., for example, expecially when the burner is shutting off or when the flow velocity of the oil is reduced. Overheating can cause undesirable cracking of the heating oil.
These disadvantages of electric resistance heating are avoided by means of the apparatus known from the German Design Application No. 78 11 098. In this apparatus, a PTC resistor with electrical current flowing through it is used to prewarm the heating oil. The PTC resistor has the property of regulating its own heat output in a known manner. This self-regulation prevents overheating of the heating oil, without expensive additional control measures being necessary.
In this known apparatus, the PTC resistor element is inserted radially into a heat-conductive, metallic sleeve which surrounds the line carrying the heating oil. The effectiveness of this prewarming apparatus is extremely poor, because on the one hand the electrical insulation necessarily disposed between the PTC resistor element and the metallic sleeve also acts as a heat resistor, while on the other hand the metallic sleeve, because of its large surface area, causes high heat losses. Finally, the metallic sleeve has a high heat capacity, so that the self-regulation of the PTC resistor element functions sluggishly and overheating of the heating oil is not reliably precluded. A further disadvantage is that the apparatus, which is placed externally on the oil supply line, requires a substantial amount of space, so that it cannot be put to use without structural alteration of the entire burner.